An acoustic well logging tool transmits acoustic pulses into the adjacent formations to obtain logs of various formation properties such as travel time and attenuation. Such a logging tool typically must have a frequency passband which is free of mechanical resonant frequency points which could distort the data obtained in the acoustic log. Most devices of this nature utilize cylindrical shell or sleeve-shaped piezoelectric transducers.
Typically, a piezoelectric transmitter transducer is constructed in the form of a hollow right cylinder. Such a cylinder can resonate in several mechanical modes. One resonant frequency is determined by the wall thickness of the cylindrical transducer; that frequency is ordinarily sufficiently high that it poses no resonance problem to the passband. Another resonant frequency is determined by the length of the right cylinder; the length can be adjusted to place the resonant point outside the passband so that minimal interference is created by that resonant point. The resonant frequency determined by the diameter of the transducer can be very near the typical passband desired for an acoustic well logging tool. Changes of diameter cannot be easily obtained; the diameter is constrained by the diameter of the body or housing of the acoustic well logging tool which is in turn limited by the diameter of the borehole in which acoustic data is typically obtained. It is not possible to increase the diameter in most instances.
It is desirable to extend the passband to lower frequency ranges. In open hole well logging, the attenuation is less for low frequency signals. Low frequency transmitted signals comprise an important portion of the data obtained in an acoustic well log.
A piezoelectric crystal has an advantage over a magnetostrictive transducer of equal size. The advantage stretches the acceptable passband for the piezoelectric crystal to lower frequencies. The circuit of this disclosure further lowers the resonant point. It moves the resonant point of the piezoelectric crystal diametric dimension lower, thereby obtaining a reduced diametric resonant frequency point, and inevitably expanding the acceptable frequency range of the transducer.
The present apparatus incorporates a switching circuit which is selectively connected across the input terminals of the piezoelectric crystal. The switching circuit is selectively switched to enable selective charging of a charging circuit wherein charge interchange occurs between a storage capacitor (in actuality, the crystal) and an inductor to initiate ringing. This interchange changes the shape of the charging curve for the circuit, and thereby changes the resonant frequency to a lower resonant frequency.